Up to this point, every font I've experienced with ligatures has
replaced the codepoints that were replaced for combining with a space.
For example, if a font has a ligature for "!=" to turn it into a glyph,
it'd shape to `[not equal glyph, space]`, so it'd still take up two
cells, allowing us to style both.
Monaspace, however, does not do this. It turns "!=" into `[not equal
glyph]` so styles like backgrounds, underlines, etc. were not extending.
This commit detects multi-cell glyphs and inserts synthetic blank cells
so that styling returns. I decided to do this via synthetic blank cells
instead of introducing a `cell_width` to the shaper result because this
simplifies the renderers to assume each shaper cell is one cell. We can
change this later if we need to.
Annoyingly, this does make the shaper slightly slower for EVERYONE to
accomodate one known font that behaves this way. I haven't benchmarked
it but my belief is that the performance impact will be negligible
because to figure out cell width we're only accessing subsequent cells
so they're likely to be in the CPU cache and also 99% of cells are going
to be width 1.
